sorting.dart

import 'dart:collection';
import 'dart:convert';

void main() {
    final data = json.decode(
      "[{\"videoId\":1,\"videoName\":\"Test Video 1\",\"duration\":300,\"createdTime\":\"1969-07-20 20:18:04Z\",\"isWatch\":1},{\"videoId\":1,\"videoName\":\"Test Video 1\",\"duration\":300,\"createdTime\":\"1969-07-20 20:18:04Z\",\"isWatch\":50},{\"videoId\":1,\"videoName\":\"Test Video 1\",\"duration\":300,\"createdTime\":\"1969-07-21 20:18:04Z\",\"isWatch\":20}]");

  final map = HashMap<String, int>();
  for (var value in (data as List)) {
    if (map[value['createdTime']] == null) {
      map[value['createdTime']] = value['isWatch'];
    } else {
      map[value['createdTime']] = map[value['createdTime']] + value['isWatch'];
    }
  }

  print(map);
}